September 6, 1900] 



NA TURE 



447 



multiplication of the cells by cleavage, and thus increase in size 

 of the embryo accompanies specialisation of structure. As the 

 process continues, the embryo gradually assumes the shape 

 characteristic of the species to which its parents belonged, 

 until at length it is fit to be born and to assume a separate 

 existence. 



The conversion of cells, at first uniform in character, into 

 tissues of a diverse kind is due to forces inherent in the cells 

 in each layer. The cell plasm plays an active though not an 

 exclusive part in the specialisation ; for as the nucleus influences 

 nutrition and secretion, it acts as a factor in the differentiation 

 of the tissues. When tissues so diverse in character as mus- 

 cular fibre, cartilage, fibrous tissues, and bone arise from the 

 cells of the middle or mesoblast layer, it is obvious that, in 

 addition to the morphological differentiation affecting form and 

 structure, a chemical differentiation affecting composition also 

 occurs, as the result of which a physiological differentiation takes 

 place. The tissues and organs become fitted to transform the energy 

 derived from the food into muscular energy, nerve energy, and other 

 forms of vital activity. Corresponding differentiations also modify 

 the cells of the outer and inner layers. Hence the study of the 

 development of the generalised cell layers in the young embryo 

 enables us to realise how all the complex constituent parts of the 

 body in the higher animals and in man are evolved by the pro- 

 cess of differentiation from a simple nucleated cell — the fertilised 

 ovum. A knowledge of the cell and of its life-history is there- 

 fore the foundation-stone on which biological science in all its 

 departments is based. 



If we are to understand by an organ in the biological sense a 

 complex body capable of carrying on a natural process, a nucleated 

 cell is an organ in its simplest form. In a unicellular animal or 

 plant such an organ exists in its most primitive stage. The 

 higher plants and animals again are built up of multitudes of 

 these organs, each of which, whilst having its independent life, 

 is associated with the others, so that the whole may act in 

 unison for a common purpose. As in one of your great factories 

 each spindle is engaged in twisting and winding its own thread, 

 it is at the same time intimately associated with the hundreds of 

 other spindles in its immediate proximity, in the manufacture 

 of the yarn from which the web of cloth is ultimately to be 

 woven. 



It has taken more than fifty years of hard and continuous 

 work to bring our knowledge of the structure and development 

 of the tissues and organs of plants and animals up to the level of 

 the present day. Amidst the host of names of investigators, 

 both at home and abroad, who have contributed to its progress, 

 it may seem invidious to particularise individuals. There are, 

 however, a few that I cannot forbear to mention, whose claim 

 to be named on such an occasion as this will be generally 

 conceded. 



Botanists will, I think, acknowledge Wilhelm Hofmeister as 

 a master in morphology and embryology, Julius von Sachs as the 

 most important investigator in vegetable physiology during the 

 last quarter of a century, and Strasburger as a leader in the study 

 of the phenomena of nuclear division. 



The researches of the veteran Professor of Anatomy in Wiirz- 

 burg, Albert von Kolliker, have covered the entire field of 

 animal histology. His first paper, published fifty-nine years ago, 

 was followed by a succession of memoirs and books on human 

 and comparative histology and embryology, and culminated in 

 his great treatise on the structure of the brain, published in 

 1896. Notwithstanding the weight of more than eighty years, 

 he continues to prosecute histological research, and has pub- 

 lished the results of his latest, though let us hope not his last, 

 work during the present year. 



Amongst our own countrymen, and belonging to the genera- 

 tion which has almost passed away, was William Bowman. His 

 investigations between 1840 and 1850 on the mucous membranes, 

 muscular fibre, and the structure of the kidney, together with his 

 researches on the organs of sense, were characterised by a power 

 of observation and of interpreting difficult and complicated 

 appearances which has made his memoirs on these subjects land- 

 marks in the history of histological inquiry, 



Oi the younger generation of biologists, Francis Mailland 

 Balfour, whose early death is deeply deplored as a loss to 

 British science, was one of the most distinguished. His powers 

 of observation and philosophic perception gave him a high place 

 as an original inquirer, and the charm of his personality — for 

 charm is not the exclusive possession of the fairer sex— endeared 

 him to his friends. 



NO. 1 6 10, VOL. 62] 



General Morphology. 

 Along with the study of the origin and structure of the 

 tissues of organised bodies, much attention has been given 

 during the century to the parts or organs in plants and animals, 

 with the view of determining where and how they take their 

 rise, the order of their formation, the changes which they pass 

 through in the early stages of development, and their relative 

 positions in the organism to which they belong. Investigations 

 on these lines are spoken of as morphological, and are to be 

 distinguished from the study of their physiological or functional 

 relations, though both are necessary for the full comprehension 

 of the living organism. 



The first to recognise that morphological relations might exist 

 between the organs of a plant, dissimilar as regards their func- 

 tion, was the poet Goethe, whose observations, guided by his 

 imaginative faculty, led him to declare that the calyx, corolla, 

 and other parts of a flower, the scales of a bulb, &c., were 

 metamorphosed leaves, a principle generally accepted by 

 botanists, and indeed extended to other parts of a plant, which 

 are referred to certain common morphological forms although 

 they exercise different functions. Goethe also applied the same 

 principle in the study of the skeletons of vertebrate animals, and 

 he formed the opinion that the spinal column and the skull 

 were essentially alike in construction, and consisted of verte- 

 brae, an idea which was also independently conceived and 

 advocated by Oken. 



The anatomist who in our country most strenuously applied 

 himself to the morphological study of the skeleton was Richard 

 Owen, whose knowledge of animal structure, based upon his 

 own dissections, was unrivalled in range and variety. He 

 elaborated the conception of an ideal, archetype vertebrate form 

 which had no existence in nature, and to which, subject to 

 modifications in various directions, he considered all vertebrate 

 skeletons might be referred. Owen's observations were con- 

 ducted to a large extent on the skeletons of adult animals, of the 

 knowledge of which he was a master. As in the course of de- 

 velopment modifications in shape and in the relative position of 

 parts not unfrequently occur and their original character and 

 place of origin become obscured, it is difficult, from the study 

 only of adults, to arrive at a correct interpretation of their mor- 

 phological significance. When the changes which take place in 

 the skull during its development, as worked out by Reichert and 

 Rathke, became known and their value had become appreci- 

 ated, many of the conclusions arrived at by Owen were 

 challenged and ceased to be accepted. It is, however, due to 

 that eminent anatomist to state from my personal knowledge of 

 the condition of anatomical science in this country fifty years 

 ago, that an enormous impulse was given to the study of com- 

 parative morphology by his writings, and by the criticisms to 

 which they were subjected. 



There can be no doubt that generalised arrangements do exist 

 in the early embryo which, up to a certain stage, are common to 

 animals that in their adult condition present diverse characters, 

 and out of which the forms special to different groups are 

 evolved. As an illustration of this principle, I may refer to the 

 stages of development of the great arteries in the bodies of 

 vertebrate animals. Originally, as the observations of Rathke 

 have taught us, the main arteries are represented by pairs of 

 symmetrically arranged vascular arches, some of which enlarge 

 and constitute the permanent arteries in the adult, whilst others 

 disappear. The increase in size of some of these arches, and 

 the atrophy of others, are so constant for different groups that 

 they constitute anatomical features as distinctive as the modifica- 

 tions in the skeleton itself. Thus in mammals the fourth 

 vascular arch on the left side persists, and forms the arch 

 of the aorta ; in birds the corresponding part of the aorta is an 

 enlargement of the fourth right arch, and in reptiles both arches 

 persist to form the great artery. That this original symmetry 

 exists also in man we know from the fact that now and again 

 his body, instead of corresponding with the mammalian type, 

 has an aortic arch like that which is natural to the bird, and in 

 rarer cases even to the reptile. A type form common to the 

 vertebrata does therefore in such cases exist, capable of evolution 

 in more than one direction. 



The reputation of Thomas Henry Huxley as a philosophic 

 comparative anatomist rests largely on his early perception of, 

 and insistence on, the necessity of testing morphological con- 

 clusions by a reference to the development of parts and organs, 

 and by applying this principle in his own investigations. The 

 principle is now so generally accepted by both botanists and 



